Gradient transparency of normal map?
Channel the gradient into a MixRGB node where you mix the normal map color with a neutral blue color (and by neutral, i mean R=0.5, G=0.5, B=1.0, like the default input of a Normal Map node). You can of course use this to blend between the two normal maps too. To blend between normal maps correctly though, you should use Split XYZ nodes, add R/G values and multiply B values between the maps, and then do Combine XYZ again.